Recently, a multimedia technology has been rapidly developed with the advent of the information-oriented era. The high performance and the portability of electronic products have been strongly demanded. Such demand inevitably requires an energy source, such as a secondary battery. Under the circumstances, a new secondary battery is actively investigated and developed as the energy source for aiming at small-sizing and increasing the capacity and the energy density. So-called lithium ion secondary batteries were commercialized early in the 1990's. Such a lithium ion secondary battery includes a metal oxide and a carbon metal having a property of capable of occluding a lithium ion as the positive electrode and negative electrode, respectively. In such a secondary battery, the positive and the negative electrodes are opposite to each other with a separator and an electrolyte interposed between them. The lithium ion second battery is one of secondary batteries having a high energy density. Since a lithium ion secondary battery uses an electrolytic solution, the battery has however a possibility of causing liquid leakage to occur and leaves a problem on the safety. Also, to prevent the liquid leakage, a metallic can or case must be used as the external container. Therefore, the battery has a difficulty in making it small in size and thin in thickness.
On the other hand, Armand et al (U.S. Pat. No. 4,303,748) already proposed a chargeable electrochemical power generator which applied with a high molecular solid electrolyte made up of a solid solution of a polyalkylene oxide and an alkali metal or an alkaline earth metal in place of using an electrolytic solution.
However, a high molecular solid electrolyte has not only an insufficient ionic electric conductivity but also a very high contact resistance with a positive electrode and a negative electrode. Moreover, the high molecular solid very high contact resistance with a positive electrode and a negative electrode. Moreover, the high molecular solid electrolyte is made up of polyethylene oxide, polypropylene oxide, etc. which are ordinary polyalkylene oxides. Such a high molecular solid electrolyte has not yet been practically employed (see, K. Murata, Electrochimica Acta., Vol. 40, No. 13-14, pages 2177-2184, 1995).
Also, to solve the above-described problems, various efforts have hitherto been made. For example, Mizoguchi et al. proposed an ion conductive solid-form composition made up of an organic high molecular compound having a dielectric constant of at least 4 (for example, polyvinylidene fluoride, polyacrylonitrile, etc.) and an organic solvent having an excellent solubility to the organic high molecular compound (see, Japanese Patent Publication Nos. 61-23945 and 61-23947). This kind of the ionic conductor is called a high molecular gel electrolyte. The ionic conductor is kept in a solid state. Therefore, the ionic conductor is sometimes called simply a high molecular solid electrolyte to avoid confusion with a conventional high molecular solid electrolyte.
It is considered that the excellent mechanical strength is maintained in the high molecular gel electrolyte by a high molecular compound which becomes a matrix and the high ionic conductivity is attained by a solution portion included in the high molecular compound as a molecular level. In this case, the material design of the high molecular material tends to be focused on a matrix. The proposal of Mizoguchi et al. has been widely accepted at present and various improvements have been made (see, Gozdz et al., Polymer-made electrolytic cell separator film and the production method thereof, U.S. Pat. No. 5,418,091).
However, various problems have yet been left on the mechanical characteristics and the heat resistance. This requires a further improvement.
To this end, Gozdz et al. (U.S. Pat. No. 5,429,891) propose a crosslinked hybrid electrolytic film and the production method thereof. In the electrolytic film, a copolymer of vinylidene fluoride and hexafluoropropylene is used as the matrix polymer. Furthermore, radiations, such as electron rays, etc., are used for the formation of the crosslinked structure. The proposal is advantageous in that by controlling the content of hexafluoropropylene in that by controlling the content of hexafluoropropylene in the copolymer, a high ionic electric conductivity and a mechanical strength are attained with a good balance and that by forming a crosslinked structure with a crosslinking agent, the mechanical strength and the heat resistance are improved.
In a secondary battery, the use of a high molecular gel electrolyte not only greatly improves the ionic electric conductivity of the electrolyte but also largely reduces the contact resistance of the electrolyte with electrodes. Thereby a polymer battery has been partially practically used which utilizes the high molecular gel electrolyte. However, various large problems have yet remained.
For example, Gozdz et al also disclose a high molecular gel electrolyte made up of the copolymer of vinyldene fluoride and hexafluoropropylene and a nonaqueous electrolyte solution dissolving a lithium salt. The high molecular gel electrolyte enables improvement about one figure higher ionic electric conductivity as compared with the ionic conductive solid-form composition proposed by Mizoguchi et al. and is excellent in the mechanical strength at room temperature. However in U.S. Pat. No. 5,418,091, the high molecular gel electrolyte is insufficient in the heat resistance and is not always sufficient in the electrolytic solution retentive property at a high temperature. Also, in the high molecular gel electrolyte disclosed in U.S. Pat. No. 5,429,891, it is considered that both the mechanical characteristics an the heat resistance are improved but the electrolytic solution retentive property is yet insufficient. However, it is considered that because radiations, such as electron rays, are used. It is necessary to remove hydrofluoric acid formed due to the radiations, which makes the production method complicated. Thus, the high molecular gel electrolyte cannot always be suitable for practical use. Furthermore, to further improve the high rate discharging characteristics and the low-temperature characteristics of a polymer battery, the ionic electric conductivity has been desired for being further improved.